Contactless ultrasonic device for dimensional inspection

ABSTRACT

An acoustic detector which includes at least one emitter unit with a solid tapered profile spike (C) associated with an element (X) for exciting the spike so as to propagate ultrasound waves in an antisymmetrical propagation mode in the spike. The device also emits waves into a surrounding gas. At least one receive unit includes a tapered profile solid spike associated with the detector for receiving the ultrasound waves.

Industry requires contactless dimensional inspection tools for measuringprofiles complementing available mechanical and optical tools. Whenmeasuring profiles, the surface to be inspected is not always highlyoptically reflective and the roughness to be measured and the area overwhich the measurement is to be effected are not always suited to anoptical technique. It is not always possible to bring a sample intomechanical contact with a micrometric feeler, simply because thestructure to be inspected is too fragile to withstand the stress appliedby the feeler.

The present invention proposes contactless means of access todimensional inspection and even measurement of roughness of opticalgrade surfaces of widely varying mechanical impedance.

The invention therefore concerns a contactless device for measuring thedistance between a surface of a read head consisting of one or moreemitters and one or more receivers consisting of electromechanicaltransducers X coupled to mechanical amplifiers C (FIG. 1). The probedobject ECH (FIG. 3) has a different mechanical impedance than the mediumin which ultrasound waves generated in the read head propagate. In thepresent invention, the propagation medium between the probed object andthe read head is the air of the atmosphere, but any other gaseous mediumcould be suitable. The mechanical amplifiers are solid spikes, generallywith a conical profile and with a bandwidth adapted to amplify impulseor harmonic ultrasound motion in an antisymmetrical guided mode, forexample a bending mode, i.e. a mode whose component of displacementorthogonal to the axis of cylindrical symmetry of the cone isantisymmetrical to that axis. The use of an antisymmetrical mode coupledto a focussing spike has the unique advantage of generating withsufficient localized directional intensity an ultrasound field CHP (FIG.2) directed toward the object to be probed. Measuring the flight time inan impulse mode or the amplitude and phase of the echo reflected fromthe probed surface in a harmonic mode measures the distance between asurface element SS (FIG. 8) of the probed object and the end of thefocussing spike.

Other aspects, aims and advantages of the present invention will becomemore apparent on reading the following detailed description of preferredembodiments of the invention given by way of non-limiting example withreference to the accompanying drawings, in which:

FIG. 1 is a diagram showing in axial section a spike C coupled to apiezoelectric element X in a cylindrical compartment Cpt, thecompartment and the spike being machined from the same metal rod tooptimize the transfer of mechanical stress between the piezoelectricelement X and the spike C,

FIG. 2 is a qualitative representation of the emission and receptiondirectionality diagram of the ultrasound field CHP generated in air bythe system from FIG. 1 when the end of the spike vibrates in a directionSV,

FIG. 3 is a diagram showing means for locally modifying the direction ofmaximum emission of a system identical to that shown in FIG. 1 bymachining the end of the cone with a bevel,

FIG. 4 is a diagrammatic representation of an embodiment in which thedirectionality of an emitter-receiver system is increased by polishingthe end of the spike,

FIG. 5 is a diagrammatic view in axial section showing maximumdirectional coupling between two systems identical to that shown in FIG.1, one of which is an emitter E and the other of which is a receiver R,

FIG. 6 is a diagrammatic view in axial section showing localized directcoupling between two systems identical to that shown in FIG. 1,

FIG. 7 is a diagrammatic view in axial section showing insidegeneratrices of conical spikes defining an acute angle α on whichdepends the position of the direct coupling area between the spikescorresponding to the shortest acoustic path linking the bases of thespikes,

FIG. 8 shows a configuration with two opposed spikes for detecting thepresence of a surface with small dimensions by indirect coupling betweenthe two spikes,

FIG. 9 is a diagram showing the general principle of measuring thedistance between a read head and the surface of a sample independentlyof the nature and temperature of the surrounding gas,

FIG. 10 is a diagrammatic perspective view of simple means for obtaininga curved reference surface with the end of the associated radius ofcurvature on the axis of cylindrical symmetry of the tapered spike,

FIG. 11 is a diagrammatic representation of a profile measuring deviceoperating in a sinusoidal mode and using a method of disturbing thecoupling field between the spikes and the probed surface by means of avertical screen interleaved between the two spikes whose distance fromthe probed surface is modulated,

FIG. 12 is a diagrammatic representation of a system with two read headsfor contactless measurement of the thickness of an object,

FIG. 13 is a wiring diagram of the electrical generator for exciting theemitter transducer, and

FIG. 14 is a block diagram of a contactless position detector system inaccordance with the invention.

Identical and similar items in more than one figure are, as far aspossible, identified by the same reference symbols.

A packet of ultrasound waves propagating in an asymmetrical propagationmode relative to the axis of cylindrical symmetry of a tapered solidspike C (typically a bending mode) is generated in the spike. Analternating polarization ferroelectric ceramic bonded to the plane baseof the cone is used for this purpose, for exampleJean-Pierre-Nikolowski, “lamb wave detector of the position of astylus”, doctoral thesis, Pierre and Marie Curie University, Feb. 2,1995. The half-angle at the apex of the spike is chosen to providesufficient bandwidth so that the packet of waves concentrated at the endof the cone has little, if any, deformation. At this end, the materialdisplacement vector has a rectilinear polarization SV perpendicular tothe straight line segment delimiting the alternating polarization of theceramic bonded to the plane base of the cone.

If the solid material of the cone is characterized by a low mechanicalattenuation and has a relatively low acoustic impedance, anon-negligible part of the mechanical vibration propagating in the spikeis transmitted into the air.

The inventive aspect of the above system lies, on the one hand, in thefocussing effect of the low-dispersion tapered profile of the spike, onthe other hand, in the use of a transverse wave to generate a radiatedbeam in air that is directional and intense in the vicinity of thespike, as shown in FIG. 2, and, finally, in the arrangement of thepiezoelectric element X and the focussing spike which make the assemblystrong at the base of the spike and assures optimum transfer of thestress generated by the piezoelectric element X into the focussingspike. FIG. 1 shows how optimum transfer of stress into the conicalspike C is achieved when the spike and the compartment Cpt containingthe piezoelectric element X are machined or cast in one piece. Theresonance of the piezoelectric element X inserted into the insulativetube MI and bonded to the plane base of the spike is strongly attenuatedby the absorbant W. The absorbant is confined in the compartment Cpt bythe stopper S, the insulative ring AI and the threaded cap CAP. Thebending mode that propagates in the spike is generated either by thepiezoelectric element, which naturally resonates in shear in thedirection of its thickness, such as a section X or Y+163° of a lithiumniobate crystal, or by conversion of a longitudinal wave into atransverse wave.

There is then at the end of the spike C a localized ultrasound sourcegenerating in the air an acoustic field CHP which can be used to measurethe roughness of a solid surface or to probe a material having a verylow acoustic impedance. Given the very low acoustic impedance of aircompared to any solid material, it is difficult to probe a solidmaterial in depth using ultrasound propagating in air. On the otherhand, ultrasound propagating in air can be used to probe in depth veryporous surfaces such as bacterial veils which is an example of a fragilestructure according to the teachings of the present invention whichallows ultrasonic inspection without physical contact, which is notpossible with a micromechanical feeler or using an optical technique.

The echo returned by the material is picked up either by the same probeor by another identical probe R. Using a second probe has a number ofadvantages. For example, it has the advantages of not disturbing echosin the emitter spike and of being able to detect in a preferentialdirection determined by the angular sensitivity of the receiving spikeof the receiver piezoelectric element. If the second probe picks up theecho returned by the material, the coupling between the two probes issaid to be indirect. There is also direct coupling between the twoprobes, however, and this depends on the orientation of one proberelative to the other one. This direct coupling is obtained when thedirection SV of mechanical vibration at the end of a spike is parallelto the direction of the maximum sensitivity of the transducer of thereceiving spike. If the spikes are opposite each other at their apex,direct coupling depends on the angle φ between the points (see FIG. 6)and on the coupling length l and the distance d between the spikes.Optimum contactless coupling is obtained when the axes are parallel, thedistance d between the spikes is a minimum distance, without the spikestouching, and the coupling length l is approximately one wavelength(FIG. 5). Beyond this coupling length, a pulse transmitted into thereceiver cone is split into two pulses because of reflection at the endof the receiver cone. If the generatrices of the spikes define an acuteangle α (FIG. 7), there is a particular value of α such that theshortest flight time enabling a packet of waves to travel from the baseof the emitter cone to the base of the receiver cone corresponds to anacoustic path necessarily passing through the ends of the spikes.

If oz denotes the axis defined by the bisector of the axes of the twocones (FIG. 7), d₀, the distance between the ends of the cones, C_(air)the speed of the waves in air, C_(dur) the speed of the transverse modein the Duralumin spike, θ the angle at the apex of the two identicalcones, α the angle between the inside generatrices of the two cones,t_(zi) (with i=1 or 2) the flight time of a packet of waves joining thetwo bases and passing through the abscissae z_(i) (i=1 or 2) and t_(max)the flight time of a packet of waves joining the two bases and passingthrough the ends, when the condition t_(max)<t_(z) implies the condition$\alpha > {{2 \cdot \arcsin}\quad {\left( \frac{C_{air}}{C_{dur}} \right).}}$

At 20° C., the limit angle α for which t_(max)=t_(z) is 12.6°. Thesignal at the terminals of the receiver transducer is at a maximum forthis limit angle α.

For the condition t_(max)<t_(z) to be satisfied over all the operatingtemperature range of the device, it is sufficient for α to be greaterthan the limit value of α corresponding to the highest workingtemperature in air.

Accordingly, direct coupling can be used to generate ultrasoundefficiently and without contact in a solid medium which can be movingrelative to the emitter. Direct coupling can also be used to detect thepresence of an object between the spikes. An important application ofoptimum direct coupling concerns acoustic thermometry. With anemitter-receiver system similar to that shown in FIG. 5, combined withpolishing the spikes to obtain a plane end (see FIG. 4) increasing thefacing surface area between the two spikes, an acoustic interferometerof small dimensions that can be used for thermometry is obtained. Thefacing surfaces at the ends of the spikes constitute a small resonantcavity whose resonant frequency depends on the temperature of the gasbetween the facing surfaces. To find the fundamental or harmonicresonant frequencies of the cavity it is sufficient to excite one of thetwo transducers using a sinusoidal voltage whose frequency is varied andto monitor the amplitude and phase of the signal a the terminals of thereceiver transducer. At the fundamental resonance, the distance dbetween the two spikes is equal to half the wavelength of longitudinalwaves in the gas. For an nth order harmonic resonance the followingequations apply: $\begin{matrix}{{\lambda = \frac{2d}{n}}{c = {{\lambda \quad f} = \frac{2{df}}{n}}}} & (1)\end{matrix}$

where λ is the wavelength in the gas, f is the resonant frequency, d isthe distance between the spikes, c is the speed of the longitudinalwaves in the gas and n is the order of the resonant frequency. If thegas in the cavity is deemed to be a perfect gas, the temperature of thegas can be deduced from the following equation M. Zemansky, R. H.Dittman, “Heat and thermodynamics”, Sixth Edition, McGraw-Hillinternational book company, 1981: $\begin{matrix}{T = \frac{c_{0}^{2}M}{\gamma \quad R}} & (2)\end{matrix}$

where M is the molecular weight of the gas (M=28.96 kg/kmol for air), c₀is the speed of the longitudinal waves at the extrapolated temperature Tat zero pressure to maintain perfect gas conditions, γ is the ratio ofthe specific heats of the gas (α=1.4 at 273 K for air) and R is theperfect gas constant (R=8.314 kJ/kmol·K).

Combining equations (1) and (2), the temperature of the gas is given byequation (3): $\begin{matrix}{T = {\frac{4d^{2}M}{n^{2}\gamma \quad R}f^{2}}} & (3)\end{matrix}$

For example, for a distance d of 331 μm, the fundamental (n=1) resonantfrequency of the cavity at 273 K is 500 kHz.

If the axes are at a non-zero angle φ, the coupling area is localized atthe end of the emitter spike in FIG. 6.

Minimum coupling is obtained if, starting from maximum direct coupling,one of the two spikes (or better still both of them) is rotated π/2about its axis.

In the case of minimum direct coupling between the two spikes (thedirection SV of maximum emission from the emitter transducer beingparallel to the direction of maximum sensitivity of the receiver,indirect coupling is obtained if the field emitted by the emitter E ispicked up by the receiver R after reflection from a surface element SSof a nearby sample ECH (FIG. 8).

This technique can be used to implement a position detector.

The wavelength of the waves in air at 1 MHz is 331 μm at 273 K. Usingelectronics for detecting the packet of waves based on detecting theenergy of the packet of waves, i.e. based on detecting the squared valueof the amplified signal, or (if the waveform is fixed as it is here fordirect coupling) based on simply triggering a comparator, it is possibleto achieve a resolution in terms of the arrival time of the packet ofwaves equal to a fraction of the pseudo-period of the packet of waves.

The vertical resolution of a position detector comprising a system oftwo spikes can be in the order of one micrometer. The lateral resolutiondepends on the inclination of the axes of symmetry of the spikesrelative to the probed surface and the size and radius of curvature ofthe ends of the spikes. It is in the order of a few tens to a fewhundreds of micrometers.

The emit or receive directionality of a spike can be greatly increasedby polishing its end to produce a locally plane surface EP (FIG. 4).

A preferred aspect of the invention is to operate the system in a pulsemode. Obviously it is equally possible to operate the system in asinusoidal mode. This mode of operation produces signal gain, inparticular if the operating frequency is a fundamental or harmonicmechanical resonant frequency of the emitter E and/or the receiver R.

The emitter and the receiver comprise a focussing spike C coupled to anelectromechanical source and the resonant frequencies are determined, onthe one hand, by the dimensions of the spike and the speed of theultrasound waves in the spike and, on the other hand, by the coupling ofthe spike to the electromechanical source X bonded to the base of thespike. The electromechanical source can fix the operating frequency ifthe mechanical resonance of the element constituting it is exploited. Toimprove the amplitude of the output signal it is of course preferablefor the emitter and the receiver to have exactly the same resonantfrequency. In a sinusoidal mode, the bandwidth of the spike is notcrucial and a geometrical shape other than a conical shape can bechosen. In a sinusoidal mode there is no question of accuratelymeasuring the propagation time of the ultrasound wave, but onlyvariation in amplitude and phase of the signal at the terminals of thereceiver relative to the excitation signal when a sample is moved up toa read head or a sample is slid under it.

There are various methods for measuring a profile. One consists inmechanically slaving the read head at a constant distance from theprofile and recording the electrical control signal as a function ofposition. Another moves the two spikes in a plane and observes thevariations in the flight time of the packet of waves. In the latter casethe accuracy of the measurement is more random if the profile of thesurface varies rapidly in spatial terms because the lateral resolutionof the system depends on the distance between the spike and the sample.

For a distance h between the spike and the sample (see FIG. 3), theradius r in the area of coupling with the probed surface is determinedby the time period dt between the reference time of arrival of thepacket of waves determined by the detection electronics and the time ofarrival of the head of the packet, as well as on the speed c_(l) of thelongitudinal waves in air, from the equation r={square root over (c_(l)²+L ·dt²+L +2+L ·dt·c_(l)+L ·h)}. Accordingly, for a period dt of 1 μs,a spike-sample distance h of 2 mm and a speed of 331 m/s, the radius ofthe interaction area contributing to the waveform at the time of themeasurement is 1.2 mm. To prevent the relatively long spike taking uptoo much room on the probed surface, it can be beneficial to cut it to abevel BS (FIG. 3). It can then be straightened slightly whilstmaintaining a maximum sensitivity direction perpendicular to the surfaceof the sample.

In a sinusoidal mode, the radius of the area of coupling with the probedsurface cannot maintain the lateral resolution of the impulse mode. Toimprove the lateral resolution the coupling field CHP between the twospikes and the probed surface can be disturbed using a screen Ecr whoseposition is modulated vertically at a frequency f₁ sufficiently far fromthe excitation frequency f₀ of the emitter spike, as shown in FIG. 11.Synchronous detection of the field disturbed at the frequency f₁ thenretains only the information on the profile of the object in theimmediate vicinity of the screen. The resolution obtained for theprofile depends on the thinness of the screen in the vicinity of thesurface of the probed object. The screen can be a glass plate PSC, forexample, with a razor blade fixed to its end. The razor blade is movedvertically by exciting the first symmetrical Lamb mode S₀ in the plateusing a ceramic rod PZT glued to the edge of the plate and excited atthe frequency f₁ of longitudinal resonance of the plate.

In a pulse mode, the fact that the speed of sound is temperaturedependent is inconvenient. From equation (2): $\begin{matrix}{{c = {{\sqrt{\frac{\gamma \quad {RT}}{M}}\quad {and}\quad \frac{\Delta \quad c}{c}} = {\frac{1}{2}\quad \frac{\Delta \quad T}{T}}}}\quad} & (4)\end{matrix}$

if the distance h_(m) between the ends of the spikes and the probedsurface is large relative to the distance d between the emitter E andthe receiver R of a reading system with two spikes, as in FIG. 3, theround trip flight time t_(gm) of a wave between the read head and theprobed surface is approximately $t_{gm} = {\frac{2h_{m}}{c}.}$

The relative variation in this flight time is easily related to therelative variation in temperature. The relative variation in themeasurement Δh_(m)/h_(m) is then deduced from the relative variation inthe temperature ΔT/T: $\begin{matrix}{\frac{\Delta \quad t_{gm}}{t_{gm}} = {{- \frac{\Delta \quad c}{c}} = {\left. {{- \frac{1}{2}}\frac{\Delta \quad T}{T}}\Rightarrow\frac{\Delta \quad h_{m}}{h_{m}} \right. = {{- \frac{1}{2}}\quad \frac{\Delta \quad T}{T}}}}} & (5)\end{matrix}$

where c is the speed of the waves in the gas and T is the temperature ofthe gas. If the read head is 0.6 mm from the surface, a temperatureincrease of +3° C. reduces the measured value of the height h_(m) by 3μm. Still with h_(m)=0.6 mm, with a distance d of 50 μm between the endsof the spikes the measured value of h_(m) is 1 μm too high.

To measure h_(m) over a time period and with micrometric accuracy, it isnecessary to correct the temperature drift of the system. This can beachieved in pulse mode, given that a vibrating spike emits in bothdirections, i.e. not only toward the probed surface but also in thedirection away from the probed surface. It is then sufficient to placeat a reference distance h_(ref) a reflective surface SR giving an echoafter a total flight time t_(ref). An ingenious way of amplifying theecho returned by the reference surface is to use a reference curvedsurface whose radius of curvature is centered at the end of the receiverspike. The curved surface can be a portion of a hollow tube or sphere.The important point here is the difficulty of having the axis ofcylindrical symmetry of the tube or the center of the sphere coincidewith the end of the spike. FIG. 10 shows one way to solve this problem:the reference curved surface is a portion of a tube subtending an anglemuch less than 180° in order not to prevent the spikes being moved closetogether. The tube portion is cut from a tube having an inside diameterD equal to the outside diameter of the cylindrical compartment Cpthousing the piezoelectric element X (see FIG. 1). One side of the tubeportion is fixed to the cylindrical base of the receiver probe, as inFIG. 1. This greatly facilitates achieving coincidence of the axis ofthe tube portion and the axis of the spike. The drawback of thisarrangement is that the reference distance h_(ref) equal to D/2 at theend of the spike decreases on moving up the axis from the receiverpoint. This problem is eliminated by creating an oblong hole in the tubeportion in the angular sector corresponding to the maximum sensitivityof the receiver.

A total flight time of a packet of waves from the base of an emitterprobe divides into a flight time t₁ associated with the flight time inthe first spike accumulated with a flight time t₂ in the second spikeaccumulated with a flight time t_(gm) in the gas toward the sample (andt_(gr) in the gas toward the reference plane). The flight times t_(m)and t_(ref) are expressed by the equations: $\begin{matrix}{{t_{m} = {{t_{1} + t_{2} + t_{gm}} = {t_{1} + t_{2} + \frac{2h_{m}}{c}}}}{t_{ref} = {{t_{1} + t_{2} + t_{gr}} = {t_{1} + t_{2} + \frac{2h_{ref}}{c}}}}} & (6)\end{matrix}$

In the case of measuring the position hm, the distance d between the twospikes is made as small as possible. In practice it is relatively simplefor this distance to be as small as one micrometer with the result thata total flight time, whether it is a reference time or a measurementtime, divides into a flight time t₁+t₂ in the spikes by direct couplingand a flight time t_(gm) or t_(gr) by indirect coupling depending onwhether it relates to the reference surface or the measurement surface.If the time origin is the time of arrival of the packet of waves due todirect coupling, measurement errors associated with variations in theflight times in the spikes can be eliminated. Equation (6) becomes:$\begin{matrix}{{{\Delta \quad t_{m}} = {{t_{m} - \left( {t_{1} + t_{2}} \right)} = {\frac{2h_{m}}{c} = t_{gm}}}}{{\Delta \quad t_{ref}} = {{t_{ref} - \left( {t_{1} + t_{2}} \right)} = {\frac{2h_{ref}}{c} = t_{gr}}}}{\frac{\Delta \quad t_{ref}}{\Delta \quad t_{m}} = {\left. \frac{h_{ref}}{h_{m}}\Rightarrow h_{m} \right. = {h_{ref}\frac{\Delta \quad t_{m}}{\Delta \quad t_{ref}}}}}} & (7)\end{matrix}$

Thus the absolute position can be measured independently of the speed c,and therefore independently of temperature, by calculating the ratioΔt_(m)/Δ_(ref). The measurement is then no longer dependent on anaccurate knowledge of h_(ref), Δt_(ref) and Δt_(m). In practice, thetime intervals Δt_(m) and Δt_(ref) are quantified using a high-frequencyclock. Then Δt_(m)=N_(m) T_(ck) and Δt_(ref)=N_(ref) T_(ck) where N_(m)and N_(ref) are the results of the integer division of Δt_(m) andΔt_(ref) by the clock period T_(ck). Given equation (4), it is easy todemonstrate that the number N_(ref) obtained at temperature T—denotesN_(ref)(T)—can be related to the number N_(ref)(T′) obtained attemperature T′ by the equation:$T = {T{\frac{N_{ref}^{2}\left( T^{\prime} \right)}{N_{ref}^{2}(T)}.}}$

If the temperature T is known the temperature T′ can be determined bymeasuring N_(ref)(T′).

FIG. 13 shows the wiring diagram of a step generator for periodicallycharging the emitter transducer X. The generator comprises twooscillators, an oscillator OSC1 which charges a reservoir capacitor C23and an oscillator OSC2 which turns on a transistor via which thepiezoelectric element X is excited. The inductor L1 and the diode D1reduce the step rise time and increase its amplitude to a value close totwice the value obtained with the voltage booster stage ETN. The voltagebooster stage comprises diodes (D2 to D21) and capacitors (C3 to C22).It is driven by the output ST1 of the oscillator OSC1. The oscillatorsare connected to symmetrical supply voltages +Vcc and −Vcc. Thefrequency of the oscillator OSC1 is in the order of 1 MHz and that ofOSC2 is in the order of 1 kHz. The capacitors C23, C24 and thetransistor T1 must be able to withstand the high voltages generated bythe stage ETN.

If two read heads are conjugated at two points (E,R) and (E₂,R₂), asshown in FIG. 12, it is possible to measure the thickness e of an objectwith a resolution of one micrometer. The distance h₀ between the tworead heads must be calibrated and stored in memory. It is deduced eitherfrom the measured flight time of a packet of waves from the emittertransducer of head 1 detected by the receiver transducer of head 2 whenthe sample whose thickness e is to be determined has not yet been placedbetween the heads or in the presence of a sample of known thickness e,deduced from the measured distances h_(m1) and h_(m2) relative to thetwo faces of the reference sample. Then: h₀=e+h_(m1)+h_(m2). A sample ofunknown thickness is then placed between the heads and the distancesh_(m1), h_(m2) between the faces of the sample and the read heads aremeasured. The thickness e of the object is obtained from the equation:

e=h ₀ −h _(m1) −h _(m2)

In the case of measuring a position, a two-stage method may be used toboost the level of the signal corresponding to indirect coupling. Duringthe first stage there is no sample and the signals registered by onehead are stored in a memory to be subtracted in a second stage fromsignals obtained in the presence of a sample. The distance h₀ betweentwo read heads must be slightly greater than the reference distancesh_(ref1) and h_(ref2) to prevent mutual coupling between the headsdisturbing their measurement window (defined for each head by the directcoupling signal and the echo from the associated reference surface).

A differential measurement technique consists in sampling the analogsignals, for example quantizing their amplitude on eight bits, andsaving them in memory for subsequent digital processing. To limit theamount of data saved, sampling of the analog signals can be restrictedto a measurement window of around 80 μs maximum for a point to referencesurface distance in air of 10 mm. For a sampling frequency of 100 pointsper microsecond, the maximum memory capacity for each head is 7.6kbytes, corresponding to a temperature range from −100° C. to +250° C.The temperature resolution of the device in a pulse mode is 0.1° C. Theresolution of the device for a position measurement is 1.7 μm at 0° C.

FIG. 14 is the block diagram of a device in accordance with theinvention providing a position detector with a resolution of onemicrometer and requiring no temperature correction, employing a readhead with two spikes. The distance h_(m) between the read head and thesurface of the sample is measured by exciting the emitter transducer ofthe emitter spike 2 by means of the step generator 1. This generates apacket of waves that are reflected at the sample 16 and detected by thetransducer of the receiver spike 3. The two spikes are oriented forminimum direct coupling. The detected signal is then amplified by anamplifier 4 and if necessary formatted before it is squared by asquaring circuit 5 and integrated twice by active integrators 61 and 62.The signal returned by the transducer of the receiver spike contains afirst packet of waves D due to residual direct coupling between thespikes and another packet of waves I due to indirect coupling betweenthe two spikes and the probed surface, delayed by a time Δt_(m)proportional to h_(m). To determine the flight time Δt_(m)quantitatively, the packets of waves D and I are isolated by means ofmonostables 71 and 72 which are triggered on a rising edge, a monostable73 triggered on a falling edge and an AND gate 81. This produces thesignals S1 whose rising edge corresponds to the arrival of the packet ofwaves D and S2 whose rising edge corresponds to the arrival of thepacket of waves I. The flight time Δt_(m) is obtained by applying theAND operator 83 to the signal S1 and the signal from the monostable 74triggered on a rising edge by the signal S2. An 80 MHz clock 9 quantizesthe duration Δt_(m) as an integer number N_(bcd) equal to the integerdivision of Δt_(m) by the clock period. The number N_(bcd) is counted byBCD counters 101, 102, 103, 104. A monostable 75 is triggered on afalling edge of the signal from the AND gate 83 to reset (RAZ) thecounters after an additional time-relay set by the monostable 76 and toproduce a bit which, when low, indicates the availability of data{overscore (AffEn)}={overscore (cs)}. A parallel interface 11 using anIntel 8255 programmable peripheral interface adapter transmits to amicrocomputer the binary word consisting of the number N_(bcd) and thebit {overscore (AffEn)}={overscore (cs)}. Direct display of the data isalso possible using a multiplexed four-digit LCD 13. Each of the fourdigits is addressed via the address bits a1 and a2 controlling a decoder15 which controls the tristate drivers 121, 122, 123, 124. The addressbits a1 and a2 are incremented via a CD4060 counter oscillator triggeredvia the AND gate 82 by a high level of the data presence bit AffEn.

What is claimed is:
 1. An acoustic detector device characterized in thatit includes at least one emitter unit including a solid tapered profilespike (C) associated with means (X) for exciting said spike to propagateultrasound waves in an antisymmetrical propagation mode in said spikeand to emit said waves into a surrounding gas and at least one receiveunit including a tapered profile solid spike associated with detectormeans for receiving said ultrasound waves.
 2. A device according toclaim 1 characterized in that the excitation means and the detectormeans comprise a respective piezoelectric element (X) in a compartment(Cpt), said spike and said compartment being machined in a single piece.3. A device according to claim 1 or claim 2 characterized in that the(or each) solid spike is made of a solid elastic material and has aconical profile.
 4. A device according to claim 1 characterized in thatit includes an emitter unit and a separate receiver unit and the taperedspikes of the two units are disposed in two essentially oppositedirections, the ends of said spikes facing each other at their apex overan axial coupling length l equal to the wavelength of the ultrasoundwaves and being separated from each other by a constant distance d overall of the coupling length so that the direction of the vibrations andthe direction of maximum sensitivity (SV) are parallel and maximumdirect coupling is obtained between the two spikes.
 5. A deviceaccording to claim 1 characterized in that it includes an emitter unitand a separate receiver unit, the tapered spikes of the two units aredisposed in two essentially opposite directions, without overlapping,and the apexes of the two spikes are at equal distances from a surfacewhose position relative to the device must be determined so that thedirections (SV) of the maximum emit and receive vibrations are paralleland maximum indirect coupling is obtained between the two spikes andsaid surface.
 6. A device according to claim 1 characterized in that itfurther includes means for measuring the flight time of a packet ofultrasound waves propagating in the spike of the emitter unit toward asmall surface (SS) of a sample (ECH) and reflected by that surfacetoward the spike of the receiver unit in order to determine the distance(hm) between said surface and said spikes.
 7. A device according toclaim 5 characterized in that it further includes means for measuringvariations in amplitude and phase of the ultrasound waves received bythe receiver unit after reflection at a surface, in a harmonic mode, todetermine the distance (hm) between said spikes and said surface.
 8. Adevice according to claim 1 characterized in that the end of the spikeor of at least one of the spikes is polished to obtain a locally planesurface (EP) at that end so that the directionality diagram of the spikeis finer and its sensitivity increased.
 9. A device according to claim 6characterized in that the flight time measuring means include a clockwhose frequency is high relative to the center frequency of the emittedultrasound wave.
 10. A device according to claim 6 characterized in thatit further includes means for determining a time at which wavesresulting from direct coupling with the emitter unit are received by thereceiver unit, this time being used as a time origin for calculating theflight time.
 11. A device according to claim 6 characterized in that itfurther includes a reflective surface (SR) at a known distance (h_(ref))from the spike of a common unit simultaneously forming the emitter unitand the receiver unit and on a side of said spike opposite that on whichthere is a sample surface whose distance from the spike is to bemeasured, and the distance (hm) between said sample surface and thespike is given by the equation: hm=h _(ref) Δtm/Δt _(ref) in which Δtmis the flight time of waves reflected at the sample surface, andΔt_(ref) is the flight time of waves reflected at the reference surface.12. A device according to claim 11 characterized in that the referencesurface (SR) is curved and has cylindrical or spherical symmetry and theaxis or center of symmetry respectively intersects or is coincident withthe axis of the spike.
 13. A device according to claim 12 wherein theexcitation means and the detector means comprise a respectivepiezoelectric element in a compartment, and the reference surface hascylindrical symmetry and comprises a portion of a tube of diameter D,the compartment of the transmit and receive unit has the same diameter Dand said tube is fixed to said compartment.
 14. A device according toclaim 5 characterized in that the axes of the spikes of the emitter andreceiver units define an angle α between them which satisfies thefollowing condition throughout the range of working temperatures: α>2arc sin (c _(gas) /c _(cone)) where C_(gas) designates the speed of thewaves in the gas, and C_(cone) designates the speed of the waves in thespikes.
 15. A device according to claim 1 for contactless measurement ofthe thickness (e) of a sample, characterized in that it comprises twomeasuring heads each including an emitter unit and a receiver unitspaced by a distance (h0) and between which said sample can be placed,in that it includes means for determining said distance (h₀) between thetwo heads by measuring the flight time of a packet of waves propagatingfrom one head to the other in the absence of the sample or by measuringthe round trip flight time of two packets of waves propagating betweeneach of the two heads and the corresponding face of a reference sampleof known thickness, and for determining the distance (h_(m1),h_(m2))between each of the two heads and the corresponding face of the sampleto be measured by measuring the round trip flight time of a packet ofwaves propagating between each head and said corresponding face of thesample to be measured, the thickness of which is calculated from theequation: e=h ₀ −h _(m1) −h _(m2).
 16. A device according to claim 10for measuring the relative temperature of a gas, characterized in thatsaid sample surface constitutes a reflective reference surface (SR) at aknown distance (h_(ref)) from the spike of a common unit forming anemitter unit and a receiver unit at the same time, in that the devicefurther includes means for determining the number of clock periods(N(T′)) corresponding to the round trip flight time between the end ofthe spike of the emitter and receiver unit and the reference circuit(SR) of a packet of waves propagating in the gas at a known temperature(T′) and for determining the number of clock periods (N(T))corresponding to the round trip flight time over the same path of apacket of waves propagating in the bas at the temperature (T) to bemeasured, and the temperature to be measured is given by the equation:T=T′(N(T′)² /N(T)²).
 17. A device according to claim 1 characterized inthat it further includes sampling and analog/digital converter means forprocessing the signal delivered by the receiver unit, memories forstoring the digitized data and a digital signal processor unit forestablishing the difference between signals delivered in two differentsituations.
 18. A device according to claim 7 for profile measurementcharacterized in that it includes means for disturbing the couplingfield between the spikes of the emitter and receiver unit and thescanned surface, said means including a screen (Ecr) whose position ismodulated at a frequency (f₁) different from the frequency (f₀) ofexcitation of the antisymmetric mode in the spike of the emitter unit(E), said modulation causing amplitude and phase modulation of theexcitation signal of the antisymmetric mode in the spike of the receiverunit (R), and the device further includes a synchronous detectionamplifier whose reference frequency is the frequency (f₁) of modulationof the position of the screen.
 19. A device according to claim 18characterized in that the screen (Ecr) is a plate to the end of which isfixed a razor blade shaped member disposed vertically between the twospikes, said plate being caused to resonate mechanically in thelongitudinal direction by a piezoelectric transducer (PZT) and at thefrequency (f₁) at which the position is modulated.